The Large Intestine

The last major part of the digestive tract, the large intestine, is shorter, but larger in diameter than the small intestine. The large intestine consists of:

• The cecum – or a “blind gut” located at the beginning of the large intestine.
• The colon – the longest portion of the large intestine.
• The rectum – the terminal portion of the large intestine.
• The anus – the last part of the digestive tract through which undigested food (faeces) passes out of the body.

 

The large intestine of the pig

 

Functions of the large intestine:

• Its main function is the absorption of water and the concentration of the liquid content of the large intestine. (Colon)
• The large intestine is a reservoir for waste or undigested materials that make up the faeces. (Rectum)
• Some digestion takes place in the large intestine. (Cecum)
• Excretion of faeces. (Anus)

Mucous is added to the remaining food in the large intestine, which acts as a lubricant to make passage easier. Muscle contractions push food through the intestines. is called the rectum.

Since most digestion and absorption of food takes place in the small intestine, the contents of the large intestine consist mainly of indigestible waste products such as cellulose, hemicellulose and lignin. No digestive juices are secreted in the large intestine, only mucous to lubricate the intestinal wall.

At the junction of the small and large intestine is the cecum where microbial fermentation occurs. In most animals, the cecum has little function. However, in animals such as horses and rabbits, the cecum is very important in the digestion of fibrous feeds.

The cecum of monogastric animals contains bacteria that digest any feedstuffs that escaped digestion earlier. The bacteria produce enzymes to continue digestion in the cecum and this digestion contributes less than 15% of the total digestion. The end products of this bacterial fermentation involve substances that are waste products for the bacteria but are of value to the animal. In this way animals with single stomachs are capable of utilising some of the cellulose ingested. Cellulose is digested into fatty acids and the bacteria is also responsible for the synthesis of the vitamin B-complex.

The anus is an opening through which undigested food passes out of the body. Food that enters the mouth and is not digested or absorbed as it passes down the digestive tract is excreted through the anus as faeces.

Accessory digestive glands and their functions

The liver

The liver is the largest gland in the body and is essential to life as it performs several functions:

• It stores glycogen. Glucose absorbed by the gut is changed to glycogen by the liver and stored there.
• It detoxifies poisons absorbed in the bloodstream.
• It stores vitamins A, D, E & K.
• It stores micro minerals (copper and iron).
• The liver helps in forming blood, especially in young animals.
• The liver secretes heparin, a substance that prevents blood from clotting (an anticoagulant).
• It is responsible for the synthesis of certain proteins such as plasma albumin, fibrinogen and prothrombin.
• It secretes bile which is collected and stored in the gallbladder until it is required in the small intestine.

Bile is a brownish-green viscous fluid containing bile salts and various organic substances. The functions of bile include:

• Bile aids in the emulsification of fat in the small intestine. Emulsification involves the breaking down of fat into smaller blood cells to make it easier for enzymes to function and digest food. Fat emulsification helps digest fats into fatty acids and glycerol that are easily absorbed by the small intestine.
• It neutralizes the acid coming from the stomach.
• It helps to create an alkaline medium in the duodenum, which is necessary for the absorption of fatty acids from the intestines.
• Bile salts help with the absorption of fat-soluble vitamins A, D, E & K.
• Bile has a binding effect on the chyme, which slows down the passage of ingesta through the alimentary canal, thereby ensuring sufficient digestion by the enzymes.
• Bile transports certain bile pigments to be broken down in the intestines and excreted.

After the bile salts have acted on the chyme, the majority of bile acids are efficiently reabsorbed from the ileum, secreted into the portal venous system, and returned to the liver in a process known as enterohepatic recirculation.

 

The liver of a pig

The pancreas

The pancreas has two functions and a double structure:

• The exocrine portion; secretes pancreatic juice
• The endocrine portion (Islets of Langerhans); secretes hormones such as insulin.

Pancreatic juice is a clear, distinctly alkaline fluid which contains many enzymes. The secretion of pancreatic juice is controlled by the hormone secretin. Digestive juices from the pancreas pass through the pancreatic duct into the small intestine. These secretions contain enzymes that are vital to the digestion of fats, carbohydrates, and proteins.

The functions of pancreatic juice include:

• The high sodium bicarbonate content of pancreatic juice neutralizes the acidic chyme from the stomach.
• The proteolytic enzymes (enzymes that break down protein) in pancreatic juice (namely, trypsin, chymotrypsin and carboxypeptidase) change proteins to peptones and peptides.
• Pancreatic lipase changes emulsified fats to glycerol and fatty acids.

Pancreatic amylase has a similar action to that of ptyalin in the saliva and converts starch to malt